Electrodeless vapor discharge lamp with auxiliary radiation triggering means



Jan. 4, 1966 w. A. MARRISON 3, 7,9 3

ELECTRODELESS VAPOR DISCHARGE LAMP WITH AUXILIARY RADIATION TRIGGERINGMEANS Filed June 1, 1962 2 Sheets-Sheet l PARALLEL LIGHT OUTPUT,

GHT SOURCE I i POWER I SUPPLY ELECTROLUMWESCENT 1y. 4 DE W4 RRE/V A.MARE/SON INVENTOR.

BYM Qi L VLMZJ A GENTS Jan. 4, 1966 W. A. MARRISON ELECTRODELESS VAPORDISCHARGE LAMP WITH AUXILIARY RADIATION TRIGGERING MEANS Filed June 1,1962 ELECTRON VOLTS 2 Sheets-Sheet 2 HSBA (APPROX) GROUND STATE5\N\PL\F\ED ENERGY LEVEL DIAGRAM FOR 500mm 14/4 R/QzE/V A MA Q/e/so/vINVENTOR.

BY Q 59% A GENTS United States Patent 3,227,923 ELECTRODELESS VAPORDISCHARGE LAMP WITH AUXILIARY RADIATION TRIGGER- ING MEANS Warren A.Marrison, Palos Verdes Estates, Califi, assignor, by mesne assignments,to Thompson Rama Wooldrldge Inc., Cleveland, Ohio, a corporation of OhioFiled June 1, 1962, Ser. No. 199,385 2 Claims. (Cl. 315248) Thisinvention relates to light sources employing vapor discharge lamps, andmore particularly to improved means for triggering vapor discharge lampsof the kind that are devoid of internal electrodes and which receiveenergy of excitation from sources that are external of the lamp.

Electrodeless vapor discharge lamps are known in which light emission isproduced through the ionizing action of electromagnetic fields on avaporizable, light radiating substance, such as a vapor confined in anenvelope, the ionizing action being affected without the aid ofelectrodes in the envelope. Such a lamp is comparatively simple instructure, is inexpensive to build and operate, and generally has arelatively long life because of the absence of electrodes. Oneapplication of a vapor discharge lamp using an alkali metal vapor is infrequency standard work utilizing atomic resonance phenomena. The lampmay be used to provide optical pumping for a gas cell of the same alkalimetal vapor as contained in the lamp to achieve highly accuratefrequency control of a radio-frequency signal. The control is attainedby detecting an error signal due to variation in signal frequency, andutilizing the error signal to correct the frequency of theradio-frequency signal.

The starting of such lamps has posed several problems. One reason forthis is that the power required to start the lamp is too great forcontinuous operation. The power required to cause the initial ionizationof gases within the lamp is enough to produce much too intenseillumination in continuous operation, with resulting rapid deteriorationof the lamp. It also produces a range of unwanted atomic transitionsresulting in inefficient use of the input power.

Another reason is that it has been found expedient to use a low powertransistor exciter oscillator circuit for the continuous operation, theoscillator circuit having insuflicient power, when operated from itsnormal supply, to produce the initial ionization.

Prior attempts to provide a satisfactory means for starting these lampshave resulted in some instances in the use of bulky and expensiveequipment. In instances where the starting equipment has beensimplified, the repeated use of such equipment has caused the lamps todeteriorate so that they become progressively more diflicult to start.

Accordingly, an object of this invention is to provide improved meansfor triggering electrodeless vapor discharge lamps of the kind referredto Without causing deterioration of the lamps.

A further object is to provide a simple, economical, auxiliary powermeans which can be used in conjunction with a low power, continuousenergy source to trigger a vapor discharge lamp, the auxiliary powermeans thereafter deenergizable without affecting the continuousoperation of the lamp.

The foregoing and other objects are realized according to the inventionin the provision of an auxiliary radiation means positioned to irradiatean electrodeless vapor discharge lamp so as to cause partial ionizationof the lamp vapors without causing any significant emission of lighttherefrom. In conjunction with the irradiation from the auxiliaryradiation means, an electromagnetic field is applied to the lamp vaporsfrom a main energizing source.

3,227,923 Patented Jan. 4, 1966 The strength of the electromagneticfield is insufficient by itself to trigger the lamp, but when combinedwith the auxiliary irradiation is sufficient to initiate light emissionby ionization of the vapors. The electromagnetic field is of suflicientstrength to maintain the light emission after the auxiliary radiation isremoved.

Thus, the auxiliary radiation source is utilized to supply a portion ofthe starting power to trigger the lamp, after which the total energy canbe reduced by removing the radiation and leaving only theelectromagnetic field to sustain the light emission. In this way, thereis achieved a reduction in the strength of the electromagnetic fieldthat is applied continuously to the lamp.

FIG. 1 is a view partly in section and partly schematic showing a vapordischarge lamp with means for energizing the lamp according to theinvention;

FIG. 2 is schematic of a circuit for heating a tungsten lamp;

FIG. 3 is a simplified energy level diagram for sodium;

FIG. 4 is a partial schematic view showing a modified form of energizingmeans for a vapor discharge lamp; and

FIG. 5 is a sectional view along line 55 showing in more detail theenergizing means of FIG. 4.

FIG. 1 illustrates one form of the invention as embodied in a lightsource intended for use with frequency control apparatus utilizingatomic resonance phenomena. However, it will become apparent that theprinciples of the invention are applicable to light sources useful inother environments, such as, for example, in the illumination of airportrunways. The light source may be one of the kind disclosed in US. Patent2,974,243. The light source 10 includes a vapor discharge lamp 12 and areflector 14 mounted within a housing 16. The interior of the housing 16is provided with an annular shoulder 18 approximately at a centralportion thereof for supporting a ring-like mounting bracket 20. Thebracket 20 has a large central opening 22, in which the reflector 14 ismounted, and several smaller openings 24 to reduce the cross section ofthe bracket 20 and thus to thermally insulate the reflector 14 from thehousing 16.

The reflector 14, which is made of an electrically conductive material,is generally funnel shaped, there being a neck portion 26 within whichthe lamp 12 is supported, and a flared portion 28 wedged within thecentral opening 22 of the bracket 26. The flared portion 28 of thereflector 14 has an inner surface 29 formed with the appropriatecurvature and a sufiiciently bright surface texture to provide a desiredreflection characteristic to the reflector 14. The inner surface 29 isgiven a parabolic curvature, for example, so that the light emitted bythe lamp 12 from regions near the focal point F of the reflector 14 willemanate from the reflector 14 as parallel rays.

The forward or flared end of the reflector 14 is formed with an annularshoulder 30 for supporting a quartz window 31 that is transparent to thelight emitted by the lamp 12. The window 31 serves in part as a dustcover for preventing foreign particles from depositing on the interiorsurface 29 of the reflector 14. Also, since quartz is a good reflectorof long wavelength radiation, the window 31 serves to maintain a uniformtemperature within the housing 16 by preventing the transmission of heatthrough the window 31. Both surfaces of the window 31 are preferablyprovided with anti-reflection coatings 32 and 34, for example ofmagnesium fluoride, so as to insure maximum transmission of the desiredwavelength of light emitted from the lamp 12. The window 31 andreflector 14 are fixed in position by means of an annular washer 36 ofheat insulating 3 material and a retaining ring 38 which is screwed intothe forward end of the housing 16.

The discharge lamp 12 has an elongated cylindrical transparent envelope40 made of glass, for example, one end portion of which is rigidlyattached to the neck portion 26 of the reflector 14 by means of anintermediate cement layer 42, such as an epoxy resin, for example. Theenvelope 40 protrudes from the neck portion 26 into the cavity formed bythe flared portion 28 of the reflector 14.

The lamp contains an quantity of a vaporizable substance 44, preferablyone of the alkali metals, such as rubidium, caesium, potassium, sodium,or lithium, which is stored on a metallic condensing member 46. The lamp12 also contains a quantity of bulfer gas, which may be one of the noblegases such as argon, neon, helium, or krypton. The primary purpose ofthe buffer gas is to restrict the motion of the alkali metal ions duringoperation, and thereby reduce Doppler broadening.

The condensing member 46, which may comprise a thin rod of substantiallysmaller diameter than that of the cylindrical envelope 40, is sealedthrough one end of the envelope 40 in axial alignment with the envelope40. When made in rod form, the condensing member 46 may have a diameterthat is /s to A the size of the outside diameter of the envelope 40. Thecondensing member 46 is formed of a material that is easily wettable bythe vaporizable substance 44. In addition, the condensing membermaterial should be an electrical conductor. When a vaporizable substance44 such as rubidium is used, the condensing member 46 may be made oftungsten, for example.

The purpose of the condensing member is more fully described in theaforementioned US. Patent No. 2,974,243. It will suflice to say that thecondensing member 46 functions to reduce noise in the lamp 12 by servingas a preferential collector of excess alkali metal vapor droplets whichwould normally condense on the envelope wall surfaces. A heat sink 66attached to the condensing member 46 maintains the temperature of thecondensing member 46 slightly cooler than the envelope 40 walls so thatthe vapor droplets condense on the member 46 rather than the envelope 40walls.

The condensing member 46 is surrounded by a heater 48 which is mountedon the neck portion 26 of the reflector 14. The heater 48 serves tomaintain the vapor pressure of the alkali metal vapor at the desiredlevel at which light emission can occur when an energizing field isapplied to the gas and vapor. The heater 48 may comprise a helical coilof insulation coated high resistance wire wound around the neck portion26. A layer 50 of heat insulation material covers the heater coil 48.The heater 48 may be connected to a source of direct current voltage,not shown, to receive its heating current.

For supplying energizing electric fields to the lamp 12 anelectromagnetic field producing element in the form of magneticinduction coil 56 is wound preferably around the end of the envelope 40opposite the end through which the condensing element 46 is sealed. Oneend of the coil 56 is spaced from the end of the condensing member 46along the length of the envelope 40 so as to leave an intermediateenvelope portion 58 that is free of both internal lamp structure as wellas external lamp structure. Furthermore, the lamp envelope 40 is positioned axially within the reflector 14 so that a substantial part of theintermediate unobstructed envelope portion 58 will be centered at thefocal point F of the reflector 14. Thus, during operation of the lamp12, a substantial portion of high intensity light emission will issuefrom the lamp 12 at the focal point P of the reflector 14 and emanatefrom the reflector 14 as parallel light rays.

One connection to the induction coil 56 is made through a terminal 60fastened and conductively connected to the flared portion 28 of thereflector 14. The other con- 4. nection to the coil 56 is made throughapertures 62 and 64 in the reflector 14 and housing 16, respectively.

The energizing electric field for the lamp 12 may be provided byconnecting the induction coil 56 in a modifled Colpitts oscillatorcircuit, as shown. The induction coil 56, comprising several turns ofwire wound around the end of the envelope 40, is connected in parallelwith two series connected capacitors 68 and 70, the latter capacitor 70being connected to a common ground. The coil 56 and capacitors 68 and 70form a tuned circuit. The junction of the capacitors 68 and 70 isconnected to the emitter 72 of a transistor 74. The collector 76 of thetransistor is grounded. The high voltage end of the capacitor 68 iscoupled through a capacitor 78 to the base 80 of the transistor 74.

Operating potentials are derived from a power supply 81 and voltagedivider network comprising two resistors 82 and 84 connected across theterminals 85a and 85b. The base 80 of the transistor 74 is maintained ata positive potential relative to the collector by connection through aninductor 86 to the junction of the resistors 82 and 84. The emitter 72is maintained at a slightly positive potential relative to the base 80by connection through an inductor 88 and resistor 90 to the high voltageend of the resistor 84.

The oscillator circuit is designed to produce an electromagnetic fieldin the coil 56 that is insufficient by itself to initiate ionization ofthe vapor Within the lamp 12. However, once the vapor is ionized to thepoint where substantial light emission is produced, by means which willnow be described, the electromagnetic field generated in the coil 56 isof sufficient strength to sustain the ionization and light emission.

To initiate ionization of the lamp 12 in acordance with the invention, aradiation producing means 92 is arranged to irradiate the lamp 12. Theradiation producing means 92 may be in the form of an auxiliary lightsoure mounted in the center of the window 31 along the axis of the lamp12. A voltage source 94 is connected through a switch 95 across theradiation producing means or auxiliary light source 92 for supplyingpower thereto. The auxiliary light source 92 may be a tungstenincandescent lamp or a gas discharge lamp such as one containing afilling of argon or helium. A requirement of the auxiliary light source92 is that it must emit light of appropriate wavelength to partiallyionize the vapor within the vapor lamp 12.

The auxiliary light source 92 is mounted at a suitable distance from thelamp 12 so that the light rays 96 given off by the auxiliary lightsource 92 and deflected by the reflector 14 converge in a region Radjacent to or coincident with the focal point P of the reflector 14.The position of the auxiliary light source 92 along the axis of the lamp12 is such that the source 92 does not obstruct any of the light raysemanating from the lamp 12 and made parallel by the reflector 14. Withthis arrangement, the parabolic reflector 14 serves the double purposeof focusing an intense beam of light from the auxiliary light source 92in the region R and of rendering parallel the light rays produced fromthe vapor at the region F.

In operation, as noted previously, the field produced in the coil 56 isnot sufficient in the absence of irradiation from the auxiliary lightsource 92 to trigger the lamp 12 into full ionization. However, uponirradiation from the auxiliary light source 92, the vapor within thelamp 12 becomes partially ionized. The ions formed thereby are thenacted upon by the fields produced in the coil 56 as follows:

When the oscillator circuit is energized the alternating magnetic fieldproduced axially of the induction coil 56 induces a circumferentialelectrical field at right angles to the magnetic field. Thecircumferential electric field is impressed upon the vapor ions thathave been produced by irradiation from the auxiliary light source 92,where upon more ions are produced through multiple collisions with'thevapor and gas molecules. In addition to the circumferential electricfield, there exists an alternating electric field between the highvoltage end of the induction coil 56 (opposite the grounded end) and thereflector 14 (which is grounded). The reflector 14 thus constitutes asecond electromagnetic field producing element, the first element beingconstituted. by the induction coil 56. Since the composite of the twoelectricfields is concentrated in the central region 58 of the lamp 12,betweenthe conidensing member 46 and the. induction coil 56, there willbe a concentration of ionization produced in this region. Accordingly,the lamp12 will be triggered into light emis-v sion, with light ofrelatively high intensity beingemit'ted from the focal point regions ofthe reflector 14.

Once the lamp 12 has been ionized sufliciently to pro duce the requiredamount of light,-the auxiliary light source 92 can be deenergized, sincethe energy required to sustain light emission is substantially less thanthat neces sary to initiate emission. The electric field produced in thecoil 56 will be adequate to sustain the light emission. It has beenfound that the magnitude of the electric field needed to start the lamp12 with the aid of an auxiliary light source 92 is at least twenty-fivepercent less than the magnitude of field required when the light source92 is not present. Since this reduced field is present during continuousoperation of the lamp 12, the life of the lamp is extended considerably.In order to maintain an electrodeless discharge in the lamp 12 by meansof high frequency currents in the coil 56 surrounding the lamp 12 it isnecessary that there be some degree of ionization of the medium withinthe envelope 40. The discharge will be maintained continuously providedthat the rate of ionization by collision induced by the high frequencyelectromagnetic field is at least equal to the recombination rate withinthe medium. It is the recombination of electrons with positive ions andthe reversion to lower energy states that is responsible for thespectral emission of a vapor lamp 12. v

, In general, once ionization has been initiated, the state ofionization may be sustained by a relatively small amount of applied highfrequency powerin the coil 56. This is because ions moving with longfree paths under the influence of the high frequency field can readily.accumulate .the energy required to create new ions through collisionprocesses.

However, the amount of high frequency field which is capable ofsustaining ionization at a low level is not sufficient to initiate adischarge without auxiliary means. It is such auxiliary means with whichthis invention is concerned.

There is some evidence that throughout matter, and in particular in themedium within the lamp envelope 40, there exist spontaneously a smallnumber of unbound electric charges. Those may be the result of cosmicradiation, or of ionization by collision through thermal motion or ofthe presence of very small radioactive impurities in some way associatedwith the'equipment.

A very small number of such charges, if acted upon by a high frequencyfield, will result in a general avalanche breakdown ofthe medium, if theintensity of the field is sufficiently increased to the point whereionization by collision exceeds the normal rate of recombination.However,.the amount of high frequency field that is required to initiatean electrical breakdown in this way causes a runawaycondition in thedischarge that results in an over-abundance of high energy transitionsand the production of many unwanted spectral components in the emittedlight as well as overheating and rapid deterioration of the lamp.

In a 'copending concurrently filed application by the same inventor asthe present application, entitled Electrodeless Vapor Discharge LampWith Auxiliary Voltage Triggering Means (STL 464), controlled means are6 disclosed for making use of a transient high energy, high frequencyexcitation to initiate an avalanche breakdown without being harmful tothe lamp.

Inthe present application, other means are utilized which have asomewhat similar end result through increasing the number of freecharges existing in the active medium of the lamp which are acted uponby the applied high frequency electromagnetic field. 1 Thefollowing'discussion is based on the use of alkali metals as the activemedium, but the same principles apply, with suitable modification, toother materials.

It is well-known that the ionization potentials for the alkali metalsrange from 5.39 electron volts to 3.89 electron volts and that theseenergies correspond to the quantum energies of light radiation from 2300angstroms to 3187 angstroms according to the following table:

Table I Ionization Threshold Alkali Metal Potential (E), Wavelength (A),

electron volts angstroms Lithium 5. 39 2, 300 5.14 2, 412 4. 34 2, 8574.18 2, 966 Oaesium 3. 89 3, 187

The above table is taken from Kaye and Laby Table of Physical andChemical Constants, Longmans, Green and Co., eleventhedition, page 184,with the threshold wavelength A being computed from the expression E\=12397.8 10 e.v.-cm., also taken from the above reference, on page 192.

As a consequence of this phenomenon, if, in the embodiment shown in FIG.1, the illumination provided by the light source 92 contains light ofwavelength equal to or less than the threshold values shown in Table I,the corresponding alkali metals will be ionized from the ground state bythe interaction of the light with the vapor. In the particular case ofrubidium, for example, some rubidium atoms will be ionized if the lightreaching the interior of the lamp envelope is of wavelength 2966angstroms or less. The actual amount of such ionization depends upon thecross section of interaction of photons with rubidium atoms, the density(and hence the temperature) of the'rubidium vapor, and upon theintensity of illumination of the stated wavelength. It will be evidentthat, by controlling the aforementioned factors, a vast increase in thenumber of free charges existing in the active medium of the lamp can beobtained which is accompanied by a significant decrease in the highfrequency excitation required to cause avalanche breakdown of the mediumand the emission of light of wavelenth characteristic of the medium.

Analogous results are obtained by the use of other alkali metals, takingaccount of the characteristic ionization potentials and thecorresponding threshold wavelengths required for ionization.

Analogous results are obtainable also by the use of material other thanthe alkali metals such as, for example, calcium, tellurium, and argon,having ionization potentials of respectively, 6.09 e.v., 6.07 e.v., and15.68 e.v.

In order to make full use of the effect just described it is necessaryto provide envelopes for the auxiliary light source 92, and for thespectral lamp 12, which are transparent to the threshold radiationsinvolved. Standard glasses, such as fuzed quartz are available fromwhich such envelopes can be made suitable for use with alkali metalvapor lamps.

If the auxiliary light source consists of a tungsten filament lamp, suchas a flashlight lamp, only a small amount of the total light is of shortenough wavelength to produce ionization even with the alkali metalswhich have the lowest ionization potentials among the elements. However,if the tungsten filament is operated in the range from 3000" K. to 3400K. there will be adequate short wave radiation to cause initialionization of caesium or rubidium which presently is of greatestconcern. In order to ionize lithium by the direct irradiation effect nowconsidered, a tungsten filament should be operated at a temperatureabove 3600 K. Because of the short lengths of time involved in eachstarting cycle for a spectral lamp, such temperatures are entirelyfeasible.

Curves showing the spectral distribution of the radiation from a blockbody for various temperatures are given on page 12 in Measurement ofRadiant Energy, edited by W. E. Forsythe, McGraw-Hill Book Co., 1937.These curves closely approximate the spectral distribution from hottungsten.

A simple means for heating a lamp filament to high temperature for shortduration is illustrated in FIG. 2.

A capacitor 97 is charged to relatively high voltage from a DC powersource 98 through a resistor 99. When a switch 100 is closed, a surge ofabnormally high current flows through the light source 92 causing it tolight to high intensity momentarily. The normal voltage of the lightsource 92 may be in the range, for example, of /s to of the supplyvoltage. The resistance of the resistor 99 is high enough to prevent thesteady flow of enough current to light the light source 92, but willcharge the capacitor 97 on the order of seconds when the switch 100 isopen. A transistor switch such as shown in the aforementioned copendingapplication could be used with some advantage.

The foregoing discussion relates to the case in which the auxiliaryradiation is of suflicient energy, i.e. sufliciently low wavelength, toactually ionize some of the active medium in a spectral lamp in a singlestep from the ground state. In this case, the operation is clearlyunderstood as described.

It has been found by experiment that it is possible to obtain aconsiderable reduction in starting power by the use of radiation oflower energy, that is, of longer wavelength than is required to ionizedirectly from the ground state. It has been found further that lighttending toward short wavelength, that is toward the blue end of thespectrum is more effective in this than is red light. Up to a certainpoint the reduction in starting power increases with the intensity ofillumination.

These observations lead to the conclusion that, before the applicationof high frequency exciting field, the number of free charges in theactive medium of the spectral lamp is increased in some degree by thepresence of light of wavelength longer than that normally associatedwith the ionization potential.

An explanation of this phenomenon is as follows, referring to the energylevel diagram of a typical metal, sodium, shown in FIG. 3. The energylevel diagram of FIG. 3 is taken from the textbook entitled ResonanceRadiation and Excited Atoms, by Allan O. G. Mitchell and Mark W.Zemansky, University Press, Cambridge, 1934, page 13, Fig 2.

It is well known that an atom can be raised from the ground state to anyof several excited states by exposure to light of specified wavelengths.Atoms so excited remain in the higher state for a short time and thenrevert to the ground state with the emission of a quantum of radiationof the same frequency. This phenomenon is known as resonance radiation.

In particular, referring to the energy level diagram for sodium, it canbe seen that atoms can be raised from the ground state to the firstexcited 3 P and 3 P states by exposure to light of wavelengths 5890 A.and 5896 A., respectively. From these levels, it requires only 3.0electron volts, corresponding to radiation of wavelength of 4133 A. orshorter to cause ionization. Due to the short lifetime in the firstexcited state, only a small proportion of the atoms can be ionized bythis process. However, the number greatly exceeds the number of ionsresulting from residual processes in a dark environment and issufficient to cause a substantial reduction in the electromagneticenergy required to excite the electrodeless discharge.

The invention was operated successfully with the fol- Coil 56 consistedof 23 turns of No. 25 enameled copper wire wound with an inside diameterof approximately 7 millimeters. The auxiliary light source 92 consistedof a 2.8 volt flashlight bulb.

According to another embodiment, an auxiliary radiation source can beprovided by incorporating a suitable radioactive material within thelamp 12. The radioactive material can be provided in addition to or inplace of the light source 92. For example, krypton is preferablyincorporated in the argon or other gas used as a buffer gas in the lamp12. The krypton 85 emits beta particles which interact with the alkalivapor molecules as well as the noble buffer gas to partially ionize thesame in a manner similar to that attributed to the auxiliary lightsource. Since krypton 85 has a half-life of 10.6 years, its useful lifeas an auxiliary radiation source should be more than adequate. Furtheradvantages of krypton 85 are that it is relatively safe and convenientto use, is chemically inert, and does not react with the rubidium vapor.

According to another embodiment shown in FIG. 4, an auxiliary radiationsource is provided in the form of an electroluminescent device 101mounted on the end of the lamp 12. The electroluminescent device 101 maycomprise a strip of translucent or transparent plastic, such aspolyethylene terephthalate, formed into a sleeve 102. The plastic sleeve102 supports, in successive layers, a transparent conductive coating104, a layer 106 of plastic embedded electroluminescent phosphor, and anopaque, light reflective, conductive coating 108. The two conductivecoatings 104 and 108 serve as electrodes which are connected through aswitch 110 across a source 112 of alternating voltage. A suitablephosphor for the layer 106 is copper activated zinc sulfide embedded inethyl cellulose or polystyrene. Such a phosphor will emit blue lightwhen subjected to an alternating electric field. However, other wellknown electroluminescent phosphors which emit light of short wavelength(suitable for assisting in the initial ionization of the spectral lamp)may be used.

When the switch 110 is closed, the source voltage 112 will impress analternating electric field across the electroluminescent device 101 tocause the latter to emit electroluminescent light. Theelectroluminescent light will partially ionize the vapor molecules inthe lamp 12, whereupon the electric field produced in the coil 56 willact on the partially ionized vapor molecules to produce the desiredlight emission from the lamp 12.

It is now apparent that the auxiliary radiation means of the inventionprovides a simplified and economical means for starting an electrodelessvapor discharge lamp, while permitting a reduction in the continuouspower requirements of the lamp.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A vapor discharge light source, comprising:

a light reflector having a predetermined focal point;

a vapor discharge lamp including a cylindrical envelope axially alignedwith said reflector, with a central portion of said envelopesymmetrically positioned at said focal point;

a vaporizable substance within said envelope the vapors of which areadapted to emit light when subjected to an electromagnetic field;

radiant energy producing means disposed adjacent to said discharge lampenvelope along the axis thereof and arranged to subject said vapordischarge lamp to radiant energy of appropriate wavelength andsufficient strength to cause a partial ionization of said vapors Withoutcausing any significant amount of light emission therefrom; and

electromagnetic field producing means arranged to subject said vapors toa concentration of electromagnetic field in said central envelopeportion,

the magnitude of said concentrated electromagnetic field beinginsuflicient, in the absence of said radiant energy, to ionize saidvapors sufiiciently to produce light emission therefrom;

the magnitude of said electromagnetic field being suflicient, in thepresence of said radiant energy, to initiate light emission byionization of said vapors and to maintain said light emission after saidradiant energy is removed.

2. A vapor discharge light source, comprising:

a light reflector having a predetermined focal point;

a vapor discharge lamp including a cylindrical envelope axially alignedwith said reflector, with a central portion of said envelopesymmetrically positioned at said focal point;

a vaporizable substance within said envelope the vapors of which areadapted to emit light when subjected to an electromagnetic field,

an auxiliary light source disposed adjacent to said discharge lampenvelope along the axis thereof and arranged to subject said vapordischarge lamp to light energy of appropriate wavelength and suflicientstrength to cause a partial ionization of said vapors without causingany significant amount of light emission therefrom; and

electromagnetic field producing means arranged to subject said vapors toa concentration of electromagnetic field in said central envelopeportion,

the magnitude of said concentrated electromagnetic field beinginsufiicient, in the absence of said light energy, to ionize said vaporssufiiciently to produce light emission therefrom;

the magnitude of said electromagnetic field being sulficient, in thepresence of said light energy, to initiate light emission by ionizationof said vapors and to maintain said light emission after said radiantenergy is removed.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESSinger, Advances in Quantum Electronics, 1961, Columbia UniversityPress, pages 96 and 97 relied upon.

GEORGE N. WESTBY, Primary Examiner.

D. E. SRAGOW, Assistant Examiner.

1. A VAPOR DISCHARGE LIGHT SOURCE, COMPRISING: A LIGHT REFLECTOR HAVINGA PREDETERMINED FOCAL POINT; A VAPOR DISCHARGE LAMP INCLUDING ACYLINDRICAL ENVELOPE AXIALLY ALIGNED WITH SAID REFLECTOR, WITH A CENTRALPORTION OF SAID ENVELOPE SYMMETRICALLY POSITIONED AT SAID FOCAL POINT; AVAPORIZABLE SUBSTANCE WITHIN SAID ENVELOPE THE VAPORS OF WHICH AREADAPTED TO EMIT LIGHT WHEN SUBJECTED TO AN ELECTROMAGNETIC FIELD;RADIANT ENERGY PRODUCING MEANS DISPOSED ADJACENT TO SAID DISCHARGE LAMPENVELOPE ALONG THE AXIS THEREOF AND ARRANGED TO SUBJECT SAID VAPORDISCHARGE LAMP TO RADIANT ENERGY TO APPROPRIATE WAVELENGTH ANDSUFFICIENT STRENGTH TO CAUSE A PARTIAL IONIZATION OF SAID VAPORS WITHOUTCAUSING ANY SIGNIFICANT AMOUNT OF LIGHT EMISSION THEREFROM; ANDELECTROMAGNETIC FIELD PRODUCING MEANS ARRANGED TO SUBJECT SAID VAPORS TOA CONCENTRATION OF ELECTROMAGNETIC FIELD IN SAID CENTRAL ENVELOPEPORTION, THE MAGNITUDE OF SAID CONCENTRATED ELECTROMAGNETIC FIELD BEINGINSUFFICIENT, IN THE ABSENCE OF SAID RADIANT ENERGY, TO IONIZE SAIDVAPORS SUFFICIENTLY TO PRODUCE LIGHT EMISSION THEREFROM; THE MAGNITUDEOF SAID ELECTROMAGNETIC FIELD BEING SUFFICIENT, IN THE PRESENCE OF SAIDRADIANT ENERGY, TO INITIATE LIGHT EMISSION BY IONIZATION OF SAID VAPORSAND TO MAINTAIN SAID LIGHT EMISSION AFTER SAID RADIANT ENERGY ISREMOVED.